separate categories for NSDate/NSDateFormatter/NSString+InternetDateTime + rfc3339String

separating the categories was so that i could take a string that already existed and use .notation to get a readable string out of what i knew to already be an rfc3339String, using the standard iOS NSDateFormatter class members to get a medium localized date for generic usage.

these are mostly conveniences for me, but i figured i'd share them back since i have found them useful, and thought perhaps others might as well.